1. Field of the Invention
This invention relates to a panel-form loudspeaker utilising a resonant multi-mode radiator, which is suitable for applications requiring thin speaker sections such as in public address loudspeakers. The speaker exhibits a conversion efficiency approaching unity so it is suitable for applications requiring high acoustic power output from the loudspeaker.
2. Discussion of Prior Art
Current loudspeakers utilise a diaphragm or similar element which is caused to move in a gross fashion in an essentially pistonic manner to create the acoustic output. The motion of the diaphragm should be in-phase across its surface so that the diaphragm moves backwards and forwards in response to the driver actuation and this is achieved, inter alia, by the nature and size of the diaphragm in relation to the frequency band over which the loudspeaker is required to operate. In these loudspeakers the diaphragm operates largely at frequencies below those at which it exhibits resonant modes (though typically they can operate above the first resonant frequency of the diaphragm by suitably damping-out this mode) and this imposes spatial and/or frequency limitations upon the loudspeaker which are undesirable. In order to raise the threshold of resonant frequencies small diaphragms are used but these are not efficient radiators at low frequencies.
There are two main kinds of loudspeaker most widely used, and both of these utilise a diaphragm driven in pistonic manner. The first of these is the electrostatic loudspeaker in which the diaphragm is driven by the charge difference experienced between the diaphragm and a rigid backplate closely spaced behind the diaphragm. Electrostatic loudspeakers are capable of yielding a high fidelity output across a wide frequency band and they are of relatively planar configuration suitable for public address applications. However they are expensive and have very low conversion efficiency which detracts from their advantages. The other established form of pistonic-diaphragm loudspeaker is the conventional dynamic loudspeaker which incorporates an edge mounted diaphragm driven by an electromechanical driver. These loudspeakers have relatively narrow bandwidth and although they are more efficient radiators than the electrostatic loudspeakers they still have low conversion efficiency. In loudspeakers of this form it is necessary to prevent destructive interference between the forward and rearward outputs of the diaphragm. This, usually requires that the diaphragm be mounted in the front face of a substantial box housing and consequently precludes flat panel formats.